A. P. Walsh

University College London, Londinium, England, United Kingdom

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Publications (55)73.54 Total impact

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    ABSTRACT: In this paper we utilize nine years of Cluster observations in the Earth's magnetotail to investigate electron pitch-angle/energy distributions. We mainly concentrate on the population of anisotropic electrons with a dominance of the parallel phase space density and energies larger than 100 eV. The energy distribution of this population strongly varies with the downtail distance (the near-Earth x > − 12RE and the tail x ∈ [−20, − 12]RE) and along the dawn-dusk direction (the midnight |y| < 10RE and the flanks |y| > 10RE). In the tail-midnight domain the electron anisotropic population is present at all energy ranges up to 20 keV, while at the tail-flank domain only the energy range below several keV is filled by this population. In the near-Earth domain the only anisotropic electrons are cold, with energies less than 1 keV. We investigate the dependence of the energy distribution of the anisotropic electron population on the system parameters for the midnight-tail domain where the main statistics is collected. The increase of the Bz GSM component of the magnetic field corresponds to an increasing energy range filled by anisotropic electrons. The increase of the electron temperature anisotropy is providedby the enhancement of the anisotropic population at all energies up to 20 keV. There is no distinct dependence of the electron anisotropic population on the plasma flow velocity. The increase of the electron temperature corresponds to shifting of the electron anisotropic population towards higher energies. We propose a simple model of the pitch-angle/energy distribution of this electron population and discuss the origin of anisotropic electrons in the magnetotail.
    Journal of Geophysical Research: Space Physics. 08/2014;
  • Annales Geophysicae 01/2014; 32(9):1093-1117. · 1.52 Impact Factor
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    ABSTRACT: 2012 Rishbeth prizewinners C Forsyth, A N Fazakerley, A P Walsh and C J Owen address the auroral acceleration region.
    Astronomy & Geophysics 12/2013; 54(6). · 0.34 Impact Factor
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    ABSTRACT: [1] We survey the properties of electron pitch angle distributions in the magnetotail plasma sheet at a distance between 15 and 19 RE from the Earth, using data from the Plasma Electron and Current Experiment (PEACE) instrument. We limit our survey to those pitch angle distributions measured when the interplanetary magnetic field (IMF) had been steadily northward or steadily southward for the previous 3 h. We find that, at sub-keV energies, the plasma sheet electron pitch angle distribution has an anisotropy such that there is a higher differential energy flux of electrons in the (anti-) field-aligned directions. Fitting the measured pitch angle distributions with both a single and two component kappa distribution reveals that this anisotropy is the result of the presence of a second, cold, component of electrons that is observed more often than not, and occurs during both the northward and southward IMF intervals. We present evidence that suggests the cold electron component has an ionospheric, rather than magnetosheath, source and is linked to the large-scale field-aligned current systems that couple the magnetosphere and ionosphere.
    Journal of Geophysical Research: Space Physics. 10/2013; 118(10).
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    ABSTRACT: The solar wind electron distribution is observed near and within 1 AU to consist of three components: a thermal core, a suprathermal halo, and a suprathermal strahl. The former two components are isotropic, while the strahl is field aligned and flows outward along the interplanetary magnetic field. The evolution of solar wind electrons with heliocentric distance is poorly understood; although the halo is thought to be formed through pitch angle (PA) scattering of the strahl, the responsible physical process has not been conclusively identified. Measurements of solar wind electrons throughout the heliosphere are required to solve this problem. We present the first observations of the suprathermal components of the solar wind electron distribution made outside 5 AU. We find indications of a strahl component narrower than that predicted by extrapolating observations and models of electrons in the inner heliosphere, suggesting the rate of electron pitch angle scattering in the solar wind can decrease with increasing heliocentric distance.
    Geophysical Research Letters 06/2013; 40(11):2495-2499. · 3.98 Impact Factor
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    ABSTRACT: A comparison of magnetotail flapping (the upand-down wavy motion) between the Earth and the two giant planets Jupiter and Saturn has been performed through investigation of the current sheet normal of the magnetotail. Magnetotail flapping is commonly observed in the Earth’s magnetotail. Due to single spacecraft missions at the giant planets, the normal is determined through minimum variance analysis of magnetometer data during multiple intervals when the spacecraft crossed through the current sheet. It is shown that indeed a case can be made that magnetotail flapping also occurs at Jupiter and Saturn. Calculations of the wave period using generic magnetotail models show that the observed periods are much shorter than their theoretical estimates, and that this discrepancy can be caused by unknown input parameters for the tail models (e.g., current sheet thickness) and by possible Doppler shifting of the waves in the spacecraft frame through the fast rotation of the giant planets.
    Annales Geophysicae 05/2013; 31:817. · 1.52 Impact Factor
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    ABSTRACT: Since launch in 2000, the four ESA Cluster spacecraft have each crossed the dayside magnetopause region thousands of times. Many previous studies presenting analysis of data from the mission, have contributed to a better understanding of the structure and dynamics of that interface and its associated boundary layers. While 2D electron pitch angle distributions (PAD) are routinely produced by the PEACE sensors on Cluster at spacecraft spin resolution (4s), the structures in this region are known to undergo changes on faster timescales than this, in response to both external drivers and internal dynamic processes. However, in certain circumstances, near-complete pitch angle distributions can be obtained at higher time resolution using Cluster burst mode data, facilitating a more detailed analysis of the particle behaviour near the magnetopause. In this paper we present an event during which the four spacecraft made outbound crossings through the low latitude boundary layer while the magnetic field orientation allowed a full pitch angle distribution of electrons to be constructed (every 1/8 s). The four Cluster spacecraft were in the 'multi-scale' formation with separations between individual pairs of spacecraft of either ~8000 or ~800 km. During the event in question, the Cluster spacecraft observed two flux transfer events (FTEs) and made a rapid (~16s) crossing of the magnetopause. The first FTE was most prominent in the C1 data a few minutes before the spacecraft crossed the magnetopause; and the second FTE was observed by C2 just before its magnetopause crossing. Additionally, C1 detected the signature associated with the second FTE in the magnetosheath, and the data from C3 show a disturbance in the low latitude boundary layer that also appears to be related to this FTE. We have utilized the high time resolution pitch angle distributions of electrons along with the high time resolution electric & magnetic data and ion distributions, to study in detail the structure of these FTEs.
    04/2013;
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    ABSTRACT: Bright aurorae can be excited by the acceleration of electrons into the atmosphere in violation of ideal magnetohydrodynamics. Modeling studies predict that the accelerating electric potential consists of electric double layers at the boundaries of an acceleration region but observations suggest that particle acceleration occurs throughout this region. Using multispacecraft observations from Cluster, we have examined two upward current regions on 14 December 2009. Our observations show that the potential difference below C4 and C3 changed by up to 1.7 kV between their respective crossings, which were separated by 150 s. The field-aligned current density observed by C3 was also larger than that observed by C4. The potential drop above C3 and C4 was approximately the same in both crossings. Using a novel technique of quantitively comparing the electron spectra measured by Cluster 1 and 3, which were separated in altitude, we determine when these spacecraft made effectively magnetically conjugate observations, and we use these conjugate observations to determine the instantaneous distribution of the potential drop in the AAR. Our observations show that an average of 15% of the potential drop in the AAR was located between C1 at 6235 km and C3 at 4685 km altitude, with a maximum potential drop between the spacecraft of 500 V, and that the majority of the potential drop was below C3. Assuming a spatial invariance along the length of the upward current region, we discuss these observations in terms of temporal changes and the vertical structure of the electrostatic potential drop and in the context of existing models and previous single- and multispacecraft observations.
    Journal of Geophysical Research 12/2012; 117(A12):12203-. · 3.17 Impact Factor
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    The Review of scientific instruments 05/2012; 83(5):059901. · 1.52 Impact Factor
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    ABSTRACT: Advancement in solar-terrestrial science is more easily achieved through the use of multi-point and multi-mission measurement over single spacecraft methods. Multi-spacecraft observations can be used to address long standing questions regarding the connection between separated regions of the Earth's magnetosphere as well as help define observed phenomena with more clarity. One such long standing question relates to the relationship between widespread dipolarisation of the inner magnetosphere and the observable phenomenon of earthward fast flows in the tail plasma sheet (often termed "bursty bulk flows", or BBFs). The former is associated with the substorm expansion phase and the latter can be associated with reconnection at the near-Earth neutral line (NENL). We used all four Cluster spacecraft, in a multi-spacecraft configuration, to detect fast flows in the magnetotail plasma sheet region. The flows were measured using a multi-instrument approach, implementing ExB drift velocity data where 3D electric field data could be reliably reconstructed from the available 2D measurements, and particle instrument data at other times. Inter-calibration was performed using statistical methods. In addition to the Cluster fast flow detections, we used both Double Star spacecraft, when available, to detect reconfiguration of the magnetic field earthwards of the position of Cluster in the plasma sheet. Moreover, we made use of the Frey & Mende (2006) substorm onset list, compiled from data collected by the IMAGE mission, to relate observations of the tail to substorm phase. These multi-mission data were gathered from the 'tail season' intervals of 2004 & 2005. This multi-year period was chosen as they were the times when the conjunctions between Cluster & Double Star were favourable in the tail and the IMAGE mission was actively observing substorm onset signatures in Earth's auroral regions. We discuss our methods and report on progress.
    04/2012;
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    ABSTRACT: Quasi-static magnetic-field-aligned electric potential drops at altitudes between 1000 and 12000 km are able to accelerate charged particles into and out of the ionosphere above the aurora. Since 2008, Cluster has made regular passes through this so-called auroral acceleration region (AAR), facilitating studies of both the temporal evolution and spatial structure of these regions. Whilst the spacecraft can pass over this region with their foot-points separated by only fractions of a degree, this still translates to 10s km in the ionosphere, and this is comparable to the scale size of some auroral arcs. Consequently, the validity of assumptions made concerning magnetic conjugacy, or that the spacecraft are passing through the same acceleration region at different times, may be severely tested and must be closely examined. In this study, we examine a number of AAR crossings by the 4 Cluster spacecraft and compare the accelerated particle spectra recorded by the different spacecraft in order to determine the likelihood of their being conjugate or passing through the same feature at different times. From this, we attempt to understand the uncertainty in determining the temporal evolution and spatial structure of quasi-static potential drops in the AAR.
    04/2012;
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    ABSTRACT: We report our findings comparing the geometric factor (GF) as determined from simulations and laboratory measurements of the new Dual Electron Spectrometer (DES) being developed at NASA Goddard Space Flight Center as part of the Fast Plasma Investigation on NASA's Magnetospheric Multiscale mission. Particle simulations are increasingly playing an essential role in the design and calibration of electrostatic analyzers, facilitating the identification and mitigation of the many sources of systematic error present in laboratory calibration. While equations for laboratory measurement of the GF have been described in the literature, these are not directly applicable to simulation since the two are carried out under substantially different assumptions and conditions, making direct comparison very challenging. Starting from first principles, we derive generalized expressions for the determination of the GF in simulation and laboratory, and discuss how we have estimated errors in both cases. Finally, we apply these equations to the new DES instrument and show that the results agree within errors. Thus we show that the techniques presented here will produce consistent results between laboratory and simulation, and present the first description of the performance of the new DES instrument in the literature.
    The Review of scientific instruments 03/2012; 83(3):033303. · 1.52 Impact Factor
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    ABSTRACT: Magnetic holes with relatively small scale sizes, detected by Cluster and TC-1 in the magnetotail plasma sheet, are studied in this paper. It is found that these magnetic holes are spatial structures and they are not magnetic depressions generated by the flapping movement of the magnetotail current sheet. Most of the magnetic holes (93%) were observed during intervals with Bz larger than Bx, i.e. they are more likely to occur in a dipolarized magnetic field topology. Our results also suggest that the occurrence of these magnetic holes might have a close relationship with the dipolarization process. The magnetic holes typically have a scale size comparable to the local proton Larmor radius and are accompanied by an electron energy flux enhancement at a 90° pitch angle, which is quite different from the previously observed isotropic electron distributions inside magnetic holes in the plasma sheet. It is also shown that most of the magnetic holes occur in marginally mirror-stable environments. Whether the plasma sheet magnetic holes are generated by the mirror instability related to ions or not, however, is unknown. Comparison of ratios, scale sizes and propagation direction of magnetic holes detected by Cluster and TC-1, suggests that magnetic holes observed in the vicinity of the TC-1 orbit (~7-12 RE) are likely to be further developed than those observed by Cluster (~7-18 RE).
    Annales Geophysicae 03/2012; 30(3):583-595. · 1.52 Impact Factor
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    ABSTRACT: The dayside magnetopause is the primary site of energy transfer from the solar wind into the magnetosphere, and modulates the activity observed within the magnetosphere itself. Specific plasma processes operating on the magnetopause include magnetic reconnection, generation of boundary waves, propagation of pressure-pulse induced deformations of the boundary, formation of boundary layers and generation of Alfvén waves and field-aligned current systems connecting the boundary to the inner magnetosphere and ionosphere. However, many of the details of these processes are not fully understood. For example, magnetic reconnection occurs sporadically, producing flux transfer events, but how and where these arise, and their importance to the global dynamics of the magnetospheric system remain unresolved. Many of these phenomena involve propagation across the magnetopause surface. Measurements at widely-spaced (Δ ∼ 5 RE) intervals along the direction of dayside terrestrial field lines at the magnetopause would be decisive in resolving these issues. We describe a mission carrying a fields and plasmas payload (including magnetometer, ion and electron spectrometer and energetic particle telescopes) on three identical spacecraft in synchronized orbits. These provide the needed separations, with each spacecraft skimming the dayside magnetopause and continuously sampling this boundary for many hours. The orbits are phased such that (i) all three spacecraft maintain common longitude and thus sample along the same magnetopause field line; (ii) the three spacecraft reach local midday when northern European ground-based facilities also lie near local midday, enabling simultaneous sampling of magnetopause field lines and their footprints. KeywordsMagnetopause–Magnetic reconnection–Solar wind–magnetosphere coupling–Cosmic vision
    Experimental Astronomy 01/2012; · 2.97 Impact Factor
  • Journal of Geophysical Research 01/2012; 117:A12203. · 3.17 Impact Factor
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    ABSTRACT: The temporal sequence of events at substorm onset requires the generation and propagation of electromagnetic waves as the system evolves from its pre- to post-onset state. Such waves offer a unique diagnostic for the dynamics of this system, and the important coupling between the equatorial magnetosphere and auroral onset dynamics in the ionosphere. We detail the ground auroral and magnetic response to both substorms and nightside auroral activations, with particular focus on characterising the space-based counterparts of the well-known onset sequence of the auroral substorm in the ionosphere. We present specific case studies and some statistics of the evolution of magnetic wave power and auroral intensity throughout the late growth phase and expansion phase of the substorm cycle. We show strong evidence that some substorm-related auroral enhancements are clearly and demonstrably linked to the evolution of ULF wave power, such as onset arc auroral beads, whilst others have a smaller ULF wave signature. We utilise multi-point multi-instrument conjunctions from THEMIS, Cluster and GOES and other platforms to probe the magnetospheric counterpart of the ground-based signatures through onset, using a range of time-series analysis techniques. In particular, we examine the implied mapping of Pi1-2 onset signatures, which often show a clear ionospheric epicentre, to dynamical processes in the magnetotail in an attempt to diagnose the plasma physics linking onset arcs to their energy sources in the tail.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: We present the results of a survey of Cluster PEACE and CIS-CODIF data taken in the 2001-2006 tail seasons, building on the work of Walsh et al. (GRL, 2011). We examine the average pitch angle distributions of protons and electrons in the magnetotail as a function of proton plasma beta, restricted to times when the magnetosphere was exposed to steady (on a 3 hour timescale) IMF conditions and focussing in particular on dawn-dusk asymmetries. We confirm that, on average, the 2 component proton plasma sheet exists duskward of the noon-midnight meridian under steady northward IMF. An associated population of cold electrons is also observed. Dawnward of the noon-midnight meridian there are no significant fluxes of the cold component of protons and much reduced fluxes of the cold electron component, implying transport across the dusk magnetopause is the dominant formation mechanism of the two component plasma sheet for both protons and electrons. Under southward IMF, dawn-dusk asymmetries in the protons are controlled by the Y component of the IMF. For the electrons higher fluxes of high energy, field-aligned, particles are observed at dusk than at dawn. This suggests a link to a duskward offset of the tail neutral line and the preferential observation of substorm-related tail signatures in the premidnight sector. We also consider the relationship between the observed particle populations and the average behaviour of the large-scale magnetotail current systems as revealed by the Curlometer.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming flux-rope-type structures. In this study we examine an event in which Cluster 2 observed a TCR while the 3 remaining Cluster spacecraft observed the underlying magnetic structure at a range of distances from the neutral sheet. The magnetic structure has a velocity of (99, 154, -31) km s-1 in GSM (|V| = 186 km s-1), an estimated size of 1.19 RE along the direction of travel and a size between 1.94 and 2.86 RE in the direction perpendicular to the current sheet. As the structure passes the spacecraft, Cluster 1 and Cluster 4 observed a bipolar signature in BZ, plasma-sheet-like plasma and field-aligned electron flows. Cluster 3 passed closest to the centre of the structure and observed two separate reductions in the plasma density (with field-aligned electron flows); these drop-outs in the plasma sheet were possibly created by the actions of X-lines. The second drop-out in the plasma sheet also includes a reversal of the ion flow, a signature consistent with the passage of a reconnecting X-line past the spacecraft. Between the X-lines, the plasma outflow from the X-lines caused an increase in pressure which led to a localised expansion of the plasma and also the observations at Cluster 1 and Cluster 4 and the TCR. Our observations do not uniquely match either of the flux rope or the flux bulge predictions although the observation of two plasma sheet drop-outs (interpreted as X-lines, one active, one dormant) with plasma-sheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.
    Annales Geophysicae 11/2011; 29(11):2131-2146. · 1.52 Impact Factor
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    ABSTRACT: Hot electron distributions are often found in Saturn's plasma sheet and have been associated with magnetic reconnection in Saturn's outer magnetosphere. Typically the electrons are at least an order of magnitude more energetic than the surrounding electron populations and abrupt transitions are observed between the two regimes. Sometimes these energetic populations are observed as part of a bimodal distribution with more typical warm plasma sheet electrons. In this poster we present case studies of some intervals in Saturn's outer magnetosphere where these hot electrons are present and examine the surrounding structure. We show significant changes in the current and plasma sheet on the timescale of hours and disruptions in the current sheet which appear to persist for more than one rotation of the plasma sheet around the planet.
    10/2011;
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    ABSTRACT: The boundary of a planetary magnetosphere is the site of mass, momentum, and energy transport. This transport produces a layer of mixed solar wind and magnetospheric plasma inside and adjacent to the boundary. In the case of Earth, the electron structure of this layer is distinctive, and has been explained by models of the layer on open magnetic field lines. In this paper we examine the electron structure of Saturn's low-latitude boundary layer (LLBL) using observations made by the Cassini spacecraft; the typical properties and variability of Saturn's LLBL are examined in a companion paper. By analyzing the relationship between the electron density and temperature measured during Cassini magnetopause crossings we demonstrate that the electron structure of Saturn's LLBL is highly variable. At some of the crossings the structure of Saturn's LLBL is similar to previously reported examples of the structure of Earth's LLBL, where the major changes in electron density and temperature clearly occur in different regions of the layer, producing a distinctive shape to the temperature-density distribution. However, at many crossings the structure of Saturn's LLBL is unlike the previously reported examples of the structure of Earth's LLBL, since they lack the same distinctive shape to the distribution. We discuss the possible explanations for these differences in the electron structure of Saturn's LLBL, and what these differences could tell us about how the solar wind interacts with a planetary magnetosphere.
    Journal of Geophysical Research 06/2011; 116(A6):6211-. · 3.17 Impact Factor